Sodium channel currents in maturing acutely isolated rat hippocampal CA1 neurones

Sodium channel currents were recorded in excised inside-out patches from immature (P_4-10) and older (P_20-46) rat CA1 neurones. Channel conductance was 16.6±0.013 pS (P_20-46) and 19.0±0.031 pS (P_4-10). Opening patterns varied with step voltage and with age. In some patches bursting was apparent at voltages positive to -30 mV. Non-bursting behaviour was more dominant in patches from younger animals. In older animals mean open time (m.o.t.) was best described by two exponentials especially in the older cells; in the immature, there were fewer cases with two exponentials. The time constant of inactivation (τ_h) estimated in ensemble averages was best described by two exponentials (τ_hf and τ_hs) in most patches from older cells. τ_hf decreased with depolarization; τ_hs increased in the range -30 to 0 mV. The voltage dependence of τ_hf in the older cells is identical to that of the single τ_h found in the younger; the results indicate a dominance of τ_hf in the younger. Patches from younger cells more often showed one apparent active channel; in such cases, m.o.t. was described by a single exponential. However, in two cases, channels showed bursting behaviour with one of these channels showing a shift between bursting and non-bursting modes. Our findings are consistent with a heterogeneous channel population and with changes in the population in the course of maturation.